The present invention generally relates to molding methods and equipment. More particularly, this invention relates to a reconfigurable vacuum screen tool and the use of such a tool in a process, such as rapid prototyping processes and limited production runs capable of producing near-net-shape articles, including fiber preforms and composite parts formed therefrom, as well as forming of materials to produce, for example, molds.
Modern manufacturing and prototype operations have created a demand for manufacturing equipment which can be readily produced and assembled for the purpose of producing prototype and short-run components by various methods. As an example, current computer-aided design (CAD) and computer-aided manufacturing (CAM) technologies (CAD/CAM) employed to develop dies used in forming processes often rely on computer simulations of the particular forming process to reduce the design time and tooling costs for the die, as well as the time necessary to verify the design of the die. The quality of the finished die directly impacts the quality of the article produced. Thus, a rapidly produced die of lesser quality may result in additional time and costs to finish the article, which therefore increases the overall time and cost required to produce a finished article. Consequently, dies that are rapidly designed to be economical for use in prototype and short production runs are often achieved as a tradeoff in die quality and life.
While traditionally directed to the production of metal articles, rapid prototyping also finds use in the production of composite articles, an example being fiber-reinforced structural composites such as carbon-carbon composites. A common method of producing fiber-reinforced composite articles involves producing a preform comprising fibers laid up to have a desired shape and held together with a binder. Such processes are also applicable to various short-fiber composites, including but not limited to carbon-carbon and fiberglass-reinforced composites. The benefits associated with rapid prototyping of composite articles include the possibility of manufacturing in niche and limited-volume markets. In addition, rapid prototyping during the development of a composite article facilitates the ability to have the article ready for prototype assembly and facilitating production line processing requirements. These advantages are particularly apparent if the composite article can be produced “near-net-shape.”
Traditional methods of producing prototype carbon-carbon articles often entail numerous steps. A typical step is a chemical vapor deposition (CVD) process performed on a preform of the desired article. CVD processes used to produce composite articles depend on the basic principles of diffusion, such that the surface area to volume ratio is a critical factor in determining the amount of processing time required to produce the article. If the composite preform can be rapidly produced at near-net-shape, the surface area of the preform exposed to the CVD environment is maximized and the volume of the preform that must be penetrated is reduced. In addition, by reducing the amount of material and energy required by the CVD process, manufacturing costs can also be reduced. Finally, if the preform is generated to near-net-shape, less machining is required after the CVD process.
In view of the above, it is advantageous in the production of composite articles if the article and its preform can be produced using fewer steps and tools. The production of composite preforms typically entail controlled and accurate deposition of a fiber material to form what will become the fiber preform, followed by a resin transfer mold (RTM) process to infiltrate the preform. The mold necessary to produce a three-dimensional preform to near-net-shape is typically expensive to produce and usually dedicated to the production of a single composite article. Accordingly, alternative methods for producing composite preforms have been sought. For example, automated methods have been developed for depositing the fiber material. One such method, known as direct composite manufacturing (DCM), is capable of generating composite parts without the use of a mold. This process is very similar to stereolithography techniques used to produce plastic articles. Another technique involves winding a continuous fiber around a mandrel whose outer surface has the desired shape of the composite article. Yet another technique involves producing preform tapes that are laid-up to generate the desired article. Finally, a technique known as programmable powder preform process (or P4) has been developed. In this process, a large perforated screen is shaped to the desired part dimensions, and a vacuum is pulled through the screen while fibers and binder are sprayed onto the screen surface with a chopper gun. In this way, large parts can be generated relatively quickly. However, a downside to the P4 process is that limited accuracy of the chopper gun prevents finely detailed control of small geometries.
The last three methods discussed above require the use of a mold that is fabricated to be specific to the article being produced. Consequently, it would be desirable if a more versatile mold were available to generate near-net-shape articles. Various tooling has been proposed for use as a rapidly-generated mold, however none appear to achieve the advantages of the P4 vacuum screen tooling. Consequently, a reconfigurable mold capable of use in the P4 process would be desirable for producing a variety of different articles.
Known reconfigurable molds include pin-generated molds that use a matrix of parallel pins aligned so that their adjacent ends approximately generate the desired surface shape of the article to be molded. Each pin must be independently adjustable from its neighboring pins. While a continuous surface can be approximated by the pins, surface details are lost as a result of the step-like different between adjacent pins. As a result, pin-generated molds have been traditionally used in research related to sheet metal forming. The pins are often positioned by turning a lead screw located beneath each pin, requiring the use of a motor associated each pin or a single motor that can be individually engaged and disengaged with each pin. Pin positioning has also been achieved through hydraulic devices. Once the pins have been adjusted to the proper height, they are locked in place. Because the pin-generated surface is not entirely smooth and will produce surface discontinuities in the sheet metal, a thick rubber sheet is often laid over the pin ends to act as an interpolator that helps to better approximate the curvature in the space between the pins.